1. Field of the Invention
This invention relates to a digital transmission method for simplifying data transmissions between a computer and terminals and to devices for practicing the method.
2. Description of the Prior Art
In general, teleprocessing networks include one or several computers serving terminals, i.e., exchanging a stream of binary signals with said terminals through transmission lines. In each network, the large number of terminals involves a large number of transmission channels and, therefore, very sophisticated and expensive equipment. In addition, it should be recalled that in addition to data signals, communications between each terminal and the associated computer, involve other logic signals. The whole set of signals to be considered can be divided into two similar groups: a transmission group (from the terminal) and a receiving group (towards the terminal). Each of said groups generally includes:
data signals, PA0 control signals such as "request to send," "ready to send," "data terminal ready," "connect data terminal to the line," etc. PA0 clock signals (if any).
If a transmission line is assigned to each of these signals, the number of wires needed is relatively large, which is, of course, a drawback.
The use of the multiplexing techniques allows the number of lines in the network to be reduced, but slows communications down, and above all, is particularly expensive when systems known in the art are used. In addition, a predetermined voltage polarity is generally used in the computers to represent one of the bit binary values and a zero voltage for the second bit value. These signals are not suited for a conventional line transmission and therefore, they must be processed prior to transmission. For more information on this subject, see the article of I. Dorros et al titled "An Experimental 224 Mb/s Digital Repeatered Line," published in the Bell System Technical Journal of September 1966. This article recommends the use of a PST code according to which the sequence of the binary signals belonging to a bit train to be transmitted is split into pairs of bits, then converted into a sequence of ternary signals defined in accordance with a conversion table.
One of the major drawbacks of the PST code is due to the fact that it is biased. In effect, the "11" and "00" pairs of bits, respectively, are converted into a positive pulse followed by a negative pulse (+-) and into a negative pulse followed by a positive pulse (-+). Therefore, the phase of the signals provided by the "11" and "00" pairs can differ by 180.degree. which could lead to a phase ambiguity at the receiving end. The error due to this ambiguity can be corrected, of course, when installing the equipment, by inverting the wires of the transmission line at one of its ends after the execution of the connection test. Nevertheless, this constraint is particularly awkward in practice. On the other hand, the "10" and "01" pairs of bits being respectively converted into + or - followed by zero and zero followed by + or -, are likely to cause a d.c. level shift if the original bit sequence exhibits an imbalance between the numbers of zeros and ones. In addition, the PST code does not permit a simple decoding of the resulting signal. And above all, this code applies only to a single binary sequence and, therefore, is not well suited for use with simple and inexpensive multiplex equipment.